I've been researching this for awhile. Is this one of the frames you are considering?
http://buildaquiltingframe.weebly.com/index.html
There are some old threads about this on the QB, and those who built this one seem happy with it.
The biggest problem that I foresee is the harp size of the sewing machine. Most of us have a harp size of only 8 inches or so. When you get the pole in there, you run out of maneuvering space for FMQ fairly quickly. I personally also don't like the idea of having to move my machine back and forth for piecing and quilting, and having my piecing machine tied up until quilting is finished. Also, for a frame setup, it's nice to have a machine that sews many more stitches per minute than most domestic machines. For me, then, it's not just a question of building a homemade quilting frame for $200, it's also a question of purchasing a machine for the setup -- around $600 for the type of machine that sews fast and has a larger harp size.
Or, do you mean build a frame for hand quilting? That is an entirely different thing!